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Across mammals, fertility and offspring survival are often lowest at the beginning and end of females’ reproductive careers. However, extrinsic drivers of reproductive success—including infanticide by males—could stochastically obscure these expected age-related trends. Here, we modelled reproductive ageing trajectories in two cercopithecine primates that experience high rates of male infanticide: the chacma baboon (Papio ursinus) and the gelada (Theropithecus gelada). We found that middle-aged mothers generally achieved the shortest interbirth intervals in chacma baboons. By contrast, old gelada females often showed shorter interbirth intervals than their younger group-mates with one exception: the oldest females typically failed to produce additional offspring before their deaths. Infant survival peaked in middle-aged mothers in chacma baboons but in young mothers in geladas. While infant mortality linked with maternal death increased as mothers aged in both species, infanticide risk did not predictably shift with maternal age. Thus, infanticide patterns cannot explain the surprising young mother advantage observed in geladas. Instead, we argue that this could be a product of their graminivorous diets, which might remove some energetic constraints on early reproduction. In sum, our data suggest that reproductive ageing is widespread but may be differentially shaped by ecological pressures.
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This content will become publicly available on July 1, 2026
Conditional benefits of social integration in wild female geladas
While strong maternal relationships have been linked with improved offspring survival in many mammals, maternal sociality appears to provide little protection against infanticidal males. Here, we evaluated whether maternal social integration predicts offspring survival to adulthood in geladas (Theropithecus gelada), a non-human primate that faces frequent alpha male takeovers coupled with high rates of infanticide. Mothers that formed stronger grooming relationships with their female and male groupmates showed higher offspring survival on average. However, when their infants experienced early-life takeovers and faced elevated infanticide risk, these survival advantages were weaker, delayed to juvenility and linked only to female–female grooming relationships. Thus, long-term social integration was not associated with reduced infanticide risk. Given this, we then examined whether females engaged in short-term social strategies that might provide more targeted protection against would-be male attackers. Following takeovers, females—particularly those with young, vulnerable infants—groomed males less frequently and prioritized deposed, protective males (i.e. their infants’ presumed fathers) over new, potentially infanticidal males. Taken together, these data suggest that gelada mothers: (i) form long-term social relationships that might improve net offspring survival and (ii) implement short-term social strategies that might protect their offspring in ways that long-term relationships cannot.
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- Award ID(s):
- 2017976
- PAR ID:
- 10650791
- Publisher / Repository:
- Royal Society
- Date Published:
- Journal Name:
- Proceedings of the Royal Society B: Biological Sciences
- Volume:
- 292
- Issue:
- 2050
- ISSN:
- 1471-2954
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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